Post-tensioned concrete is widely used in building structures as it provides a light, structurally efficient, durable solution for the construction of commercial office buildings, residential apartments, high-rise condominiums, and mixed-use facilities such as hotels and casinos. Post-tensioned concrete is also utilized frequently in the elevated concrete decks and sometimes in the ground floor slabs of condominium structures, including the associated parking garages. Longer, thinner slabs result in greater design flexibility and require less reinforcing steel to achieve the same strength as other methods of construction. Additionally, post-tensioned concrete greatly reduces the floor-to-floor height when compared to structural steel options, which results in significant savings in the façade, HVAC, electrical, plumbing, and vertical transportation systems. As such, post-tension concrete was introduced into residential high-rise construction as a means to reduce the thickness of the elevated concrete decks, to widen spans between column supports, and to speed up the construction process.
Generally, post-tension reinforcement includes stranded cables, also referred to as “tendons” or a “strand,” that are contained typically in a plastic sheath and are positioned in the forms before the concrete is poured. Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or “tensioned,” and anchored against the outer edges of the concrete. Concrete has a very high compressive strength, typically 3,000 to over 10,000 psi, but very low tensile strength, typically 300 to 400 psi. Post-tension cables provide tensile strength to the concrete. This increase in tensile strength is achieved when the cables are stressed under significant tension loads following the pouring of the concrete.
In the past, a post-tension slab was typically constructed with cables laid out in two directions, typically at 90 degrees to one another. The cables are enclosed in a plastic sheathing to separate them from, and allow for movement within, the surrounding concrete. Each cable is anchored at one end of the slab and then placed loosely across the slab to another steel anchor located at the opposite end of the slab. Two steel reinforcing bars are placed behind the anchors to help distribute the high tensions forces from the anchors. Once the steel, cables, and anchors are installed, the concrete is poured. After the concrete has reached an acceptable strength, the cable is tensioned within the concrete slab, which is achieved by pulling the cable to a force of approximately 33,000 pounds. Two steel wedges are placed around the cable and within the anchor to lock the cable at the applied tension load.
Many early post-tension installations did not include corrosion protection of the steel anchorage and cable ends. As a result, penetration of moisture through the concrete often results in the corrosion of the post-tension cable, anchor, and wedges. The presence of corrosion can have a substantial impact on the structural integrity of the post-tension system. Additionally, a post-tension cable can be compromised when types of restoration, renovation, or remedial work are being performed on a post-tensioned reinforced structure. Moreover, post-tension cables can also be compromised when the original construction was performed poorly or maintenance of the exterior of a building is not performed on a regular basis. In any of these situations, the ends of the cables at the edges of concrete are allowed to corrode, and without proper maintenance, the corrosion can advance to the point where cables or end anchors fail.
Repair of a damaged post-tension cable is performed by typically exposing one or both ends of the damaged cable at the edges and at one or multiple locations where the cable breeched the concrete. A new cable or portion of a cable is used to replace the damaged cable. If the entire cable is replaced, the old cable is removed from the plastic sheathing within the concrete and a new cable is installed through the sheathing. The affected sheathing and the concrete are then repaired, and allowed to cure. The cable is then tensioned and the ends patched. If only a portion of a cable is replaced, the connection of the new cable to the older cable is accomplished with a post-tension cable splice and/or coupler and then repaired in the same fashion as replacing the entire cable mentioned above.
Due to the extremely high force in the cables, removal of the concrete behind the anchor and bars can cause a significant failure of the slab. As a result, replacement of deteriorated anchors requires de-tensioning of the affected cable. It should be noted that only the damaged portion of the cable is de-tensioned during repairs, as the remaining cable must retain post-tension force in order to maintain the structural integrity of the slab. The de-tensioning of the cable is accomplished by opening a small area of the concrete slab inward from the area to be repaired. Within this opening, a temporary cable “lock-off” device is installed. The lock-off location is generally two to three feet from the edge so that the cable forces can flow or be redirected around the opening with a risk of a concrete failure. The outer portion of the cable is then released, and repairs can begin.
During concrete repairs, replacement anchors and cables are installed. To provide protection against corrosion, plastic sheathed anchorage systems that protect all the parts of the anchors and cables are used. The cables are usually encased in a flexible plastic protective hose, typically called a sheath or duct, to prevent the cable from bonding to the concrete during placement and curing of the concrete. The protective sheathing remains in the structure. In some cases, the void between the cable and the sheath is filled with grout. In this manner, the cable becomes bonded to the concrete section and corrosion of the steel cable is prevented. In other cases, the cable is coated with grease prior to placement into a protective sheathing. Cables of this type are not pressure grouted after stressing. This type of post-tensioning is usually referred to as an un-bonded post-tensioning system. Once installed, and the new cable is spliced to the lock-off cable, concrete is poured and allowed to cure.
A tendon anchorage and mounting means is disclosed in U.S. Pat. No. 3,936,256 (Howlett et al.). Howlett discloses an anchor for a concrete pre-stressing tendon that is arranged at the edge of a concrete slab. Howlett further discloses a member that blocks the entry of concrete into the end of the mounting means of the anchor. However, Howlett does not disclose a member that shields a post-tension cable splice coupler from corrosion and blocks entry of concrete therein. Instead, Howlett discloses a member that shields an anchor that is fixed (i.e., does not move) within or about a concrete slab.
An apparatus and method for sealing an intermediate anchor of a post-tension anchor system is disclosed in U.S. Pat. No. 6,381,912 (Sorkin). Sorkin discloses an apparatus specifically for post-tensioning systems having intermediate anchorages. In many post-tension systems, the length of the concrete slab is too long to tension with a single anchor. In these systems, an intermediate anchor is interposed between a live end and a dead end anchor. Sorkin further discloses a sheathing system for the intermediate anchor that prevents the intrusion of liquid therein. However, Sorkin does not disclose a member that shields a post-tension cable splice coupler from corrosion and blocks entry of concrete therein. Instead, Sorkin discloses a member that shields an anchor that is fixed (i.e., does not move) within or about a concrete slab.
Thus, there is a long felt need for an apparatus that shields a post-tension cable splice coupler from the intrusion of moisture and concrete therein, thereby preventing corrosion and also allowing the splice coupler to displace relative to the concrete once the concrete has set.